OBJECTIVETo describe the clinicopathologic features and immunophenotypes of carcinoid tumorlets in the lung with bronchiectasis, and to study the morphogenesis of these tiny tumors.

METHODSThe histopathologic characteristics of 3 bronchiectasis cases with carcinoid tumorlets, 11 bronchiectasis and 2 normal lungs were studied. Specific markers of the tiny tumors and the number of neuroendocrine cells (NECs) in the airway mucosa were immunohistochemically detected by EnVision method.

RESULTSThe tumorlets in the lungs presented as multi-focal nodules and most were displayed only under microscopy. These cells were arranged in clusters and foci of fascicles which were situated in the surrounding bronchial wall and bronchioles adjacent to bronchiectatic lesion, or in the scar tissues. The tiny tumors were consisted of short fusiform cells and small ovoid cells. Their nuclei were circular, oval or long fusiform and the cells were strongly argyrophilic on Grimelius staining. Intensive positive immunostaining for calcitonin, chromogranin A, NSE and gastrin were detected. Weak positive for CK, EMA, S-100 and focal positive for HC, ACTH, 5-HT were also observed. Proliferative NECs in airway mucosa adjacent to the tiny tumors increased significantly in number, compared with those in the airway mucosa of bronchiectasis without tumorlets and normal lungs (P < 0.001, respectively).

CONCLUSIONSThe clinicopathologic features and immunophenotypes of carcinoid tumorlets resemble carcinoid tumors. They are the early stage of carcinoid development. Their development may be related to the chronic pulmonary damage resulting in hypoxia and stimulating the proliferation of NECs. These pulmonary carcinoid tumorlets can be used as a model to study the tumorigenesis of carcinoid carcinoma of the lung.